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<head>
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  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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<style>
nav{
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
  display: flex;
}
a{
  text-decoration: none;
  color: black;
  width: 100px;height: 50px;
  background: #c0c0c0;
  display: block;
  text-align: center;
  line-height: 50px;
  margin: 10px;
}
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    <a href="./倒计时案例.html" target="_blank">倒计时</a>
    <a href="./时钟.html" target="_blank">时钟</a>
    <a href="https://gitee.com/tian158/task" target="_blank">仓库地址</a>
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</body>

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